1. Field of the Invention
This invention relates to neural tissue stimulation techniques, and more particularly relates to techniques for providing more effective vagus nerve stimulation and for controlling or preventing epileptic seizures with minimized effect on the heart.
2. Description of Related Art
Epileptic seizures are the outward manifestation of excessive and/or hypersynchronous abnormal activity of neurons in the cerebral cortex. Many types of seizures occur. The behavioral features of a seizure reflect function of the portion of the cortex where the hyper activity is occurring. Seizures can be generalized and appearing to involve the entire brain simultaneously. Generalized seizures can result in the loss of conscious awareness only and are then called absence seizures (previously referred to as xe2x80x9cpetit malxe2x80x9d). Alternatively, the generalized seizure may result in a convulsion with tonic-clonic contractions of the muscles (xe2x80x9cgrand malxe2x80x9d seizure). Some types of seizures, partial seizures, begin in one part of the brain and remain local. The person may remain conscious throughout the seizure. If the person loses awareness, the seizure is referred to as a complex partial seizure.
A number of techniques are known to treat seizures including, for example, drug therapy, drug infusion into the brain, electrical stimulation of the brain, electrical stimulation of the nervous system, and even lesioning of the brain. U.S. Pat. No. 5,713,923 entitled Techniques of Treating Epilepsy by Brain Stimulation and Drug Infusionxe2x80x9d generally discloses such techniques in the background section and specifically discloses techniques for drug infusion and/or electrical stimulation to treat epilepsy. This patent is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,025,807 entitled xe2x80x9cNeurocybernetic Prosthesisxe2x80x9d and its parentage (U.S. Pat. Nos. 4,867,164 and 4,702,254) (all three patents are collectively referred to herein as the xe2x80x9cZabara patentsxe2x80x9d) disclose techniques for electrical stimulation of the vagus nerve. These Zabara patents generally disclose a circuit-based device that is implanted near the axilla of a patient. Electrode leads are passed from the circuit device toward the neck and terminate in an electrode cuff or patch on the vagus nerve.
The neuro-cybernetic prosthesis (NCP) is the primary vagus nerve stimulation (VNS) system that is presently available. This presently available VNS treatment technique for the treatment of epilepsy, however, has limited therapeutic efficacy and exerts clear but variable chronotropic effects on the human heart. See Handforth et. al., xe2x80x9cVagus Nerve Stimulation Therapy for Partial Onset Seizures: A Randomized Active Control Trial,xe2x80x9d J. Neurology, Vol. 51, pp. 45-55 (1998); Han et al, xe2x80x9cProbable Mechanisms of Action of Vagus Nerve Stimulation in Humans with Epilepsy: Is the Heart the Window into the Brain?xe2x80x9d AES Proceedings, p. 83 (1997); Frei et al., xe2x80x9cEffects of Vagal Stimulation on Human EEG,xe2x80x9d AES Poceedings, p. 200 (1998). With regard to the heart, vagus nerve stimulation has the side-effect of altering the heart rate. See Frei et al. xe2x80x9cEffects of Vagal Stimulation on Human ECG,xe2x80x9d Abstract from the Annual Meeting of the American Epilepsy Society, Vol. 39, Supp. 6 (1998), which is incorporated herein by reference in its entirety. Typically, activation of the device and stimulation of the vagus nerve causes the heart to experience a significant drop in heart rate. For example, FIG. 1A is graph illustrating the effects of vagus nerve stimulation on the heart rate for a patient. In this Figure, the horizontal axis represents time and the vertical axis represents the normalized heart rate. A value of 1 in this graph indicates that the instantaneous heart rate (IHR) at that point in time is equal to the median IHR for the current vagus nerve stimulator (VNS) device cycle (i.e., for the current 5xc2xd minute window). The graph shows that during vagus nerve stimulation from time 0 to 50, the heart rate drops to as low as 0.8 of its background rate. Similarly, FIG. 1B is a graph of the instantaneous heart rates (defined herein) of a patient as a function of time over an 8 hour period. The sharp drops that occur periodically along the bottom of the graphed line correspond to times when the vagus nerve stimulation device is reset or turned xe2x80x9conxe2x80x9d. These sharp drops illustrate the effect that vagus nerve stimulation has on the heart. Notably, the Zabara patents recognize that the heart rate slows as a result of the stimulation. This effect that vagus nerve stimulation has on the heart is undesirable due to negative short- or long-term effects on the patient. For example, the heart may become less adaptable to stresses due to the vagus nerve stimulation, which may lead to arrhythmia, asystole (heart stoppage), and possibly even to sudden death. See Asconape et al, xe2x80x9cEarly Experience with Vagus Nerve Stimulation for the Treatment of Epilepsy; Cardiac Complications, AES Proceedings, p. 193 (1998).
The relative lack of efficacy and the adverse effects of the VNS are attributable in part to inadequate stimulation. Specifically, the NCP does not change the EEG. See Salinsky et al. xe2x80x9cVagus Nerve Stimulation Has No Effect on Awage EEG Rythms in Humans,xe2x80x9d J. Epilespia, Vol. 34 (2), p.299-304 (1993). Adequate stimulation of the vagus nerve induces either synchronization or desynchronization of brain rhythms depending on the stimulation parameters used. See Michael H. Chase et al., xe2x80x9cAfferent Vagal Stimulation: Neurographic Correlates of Induced EEG Synchronization and Desynchronization,xe2x80x9d Brain Research pp. 236-249 (1967); Chase et al, xe2x80x9cCotical and Subcortical Patterns of Response to Afferent Vagul Stimulation,xe2x80x9d Experimental Neurology, Vol. 16, pp. 36-49 (1966). EEG desynchronization requires selective activation of slow conducting nerve fibers. This state of desynchronization does not favor the occurrence of seizures and is therefore preferred for this specific therapeutic purpose. The absence of EEG changes in humans during VNS suggests stimulation is inadequate and this in turn may explain its relatively low therapeutic value. See Handforth et. al., xe2x80x9cVagus Nerve Stimulation Therapy for Partial Onset Seizures: A Randomized Active Control Trial,xe2x80x9d J. Neurology, Vol. 51, pp. 45-55 (1998).
In addition, VNS provides non-selective bi-directional nerve fiber activation. In general, the VNS stimulation affects the brain (a desirable target) and also the viscera, including the heart (undesirable targets). Accordingly, VNS causes alterations in the heart EKG. Given the shape of the pulse, its biphasic nature and the intensity settings available in the NCP, selective stimulation of slow conducting nerve fibers (a necessary condition for EEG desynchronization) is highly unlikely with this device.
Further, the NCP provides indiscriminate timing for stimulation of the heart. Cardiac arrest can result from stimulation of the heart during vulnerable phases of its cycle. See Jalife J, Anzelevitch C., xe2x80x9cPhase resetting and annihilation of pacemaker activity in cardiac tissue,xe2x80x9d Science 206:695-697 (1979); Jalife J, Anzelevitch C., xe2x80x9cPacemaker annihilation: diagnostic and therapeutic implications,xe2x80x9d Am. Heart J. 100:128-130 (1980); and Winfree AT., xe2x80x9cSudden Cardiac Death: A Problem in Topology,xe2x80x9d Sci Am 248:144-161 (1983). VNS can cause cardiac arrest because the timing of stimulation does not take into account the phase or state of the cardiac cycle.
Accordingly, it is an object of the invention to provide a technique for controlling or preventing epilepsy via stimulation of the vagus nerve with minimized effect on the heart rate. It is another object of the invention to provide a technique for adjusting the vagus nerve stimulation to minimize its affect on the heart rate. It is another object of the invention to provide stimulation of the vagus nerve while maintaining the heart rate at a preset rate. It is a further object to minimize the risk of cardiac arrest in patients receiving VNS by delivering stimuli at times in the heart cycle which cause no or minimal adverse effects on rhythms generation or propagation. Other objects of the present invention will become apparent from the following disclosure.
The present invention discloses techniques for treating epilepsy by providing electrical stimulation of the vagus nerve to induce therapeutic EEG changes with little or no potentially serious or life-threatening side-effects, especially to the heart. Accordingly, the present invention discloses techniques for adjusting the vagus nerve stimulation and/or controlling the heart rate during vagus nerve stimulation to maintain the heart within desired parameters. In a preferred embodiment of the present invention, treatment is carried out by an implantable signal generator, one or more implantable electrodes for electrically stimulating a predetermined stimulation site of the vagus nerve, and a sensor for sensing characteristics of the heart such as heart rate. The heart rate information from the sensor can be used to determine whether the vagus nerve stimulation is adversely affecting the heart. Once threshold parameters are met, the vagus nerve stimulation may be stopped or adjusted. In an alternative embodiment, heart EKG signals may be monitored and applied to an EKG algorithm to detect epileptic seizures and to responsively trigger the signal generator to provide stimulation to the vagus nerve.
In an alternative embodiment, the invention may include a modified pacemaker to maintain the heart in desired conditions during the vagus nerve stimulation. In yet another embodiment, the invention may be simply a modified pacemaker having circuitry that determines whether a vagus nerve is being stimulated. In the event that the vagus nerve is being stimulated, the modified pacemaker may control the heart to maintain it within desired conditions during the vagus nerve stimulation.
In yet another embodiment, EKG rhythms may be sensed so as to minimize EKG changes via cybernetic techniques. In another embodiment, the present invention may selectively stimulate certain fiber groups within the vagus nerve to block the propagation of impulses towards the viscera, such as the heart, using electrophysiologic techniques. In still another embodiment, the present invention may sense brain EEG to provide feedback on the vagal nerve stimulation. Alternatively, heart EKG may be monitored to determine whether there is a risk of a possible seizure onset to either adjust the VNS stimulation or to warn the patient.